LAB WEEK 10: METAMORPHIC ROCKS IN HAND SAMPLE

Geology 2312: Petrology
Name
LAB WEEK 10: METAMORPHIC ROCKS IN HAND SAMPLE
Knotty staurolite-garnet schist, Picuris Range, New Mexico
Purpose: This week you will be learning how to identify and name metamorphic rocks in hand sample.
For each rock, you need to identify the minerals present (if visible), identify the rock's textural features,
and give the rock a name. In the space provided on the attached worksheet, identify any textures or
fabrics that you observe, then give as complete a rock name as possible, including metamorphic minerals
and a textural or compositional name. Refer to the metamorphic textural classification handout as a
guide. There will also be a set of reference metamorphic rock samples to help you become familiar with
the various types of metamorphic rocks.
The nomenclature for metamorphic rocks is archaic! Rock names are inherited from all manner of early
descriptive terms, some referring to minerals, some to texture, and some to composition or color, and
they are difficult to remember. But, in simplest terms, metamorphic rock names are assigned as follows:
1. Basis of metamorphic classification is on —
a. fabric or texture
(e.g., schist, gneiss, mylonite, hornfels, etc.)
b. minerals
(e.g., amphibolite, etc.)
c. unusual composition
(e.g., quartzite, marble, serpentinite, eclogite, etc.)
d. color
(e.g., blueschist, greenstone/greenschist, whiteschist, etc.)
2. Rock names should be given as "mineral(s) + texture" (hence, muscovite-garnet schist) —
a. list minerals in increasing order of abundance (e.g., muscovite-garnet)
b. provide a textural name (e.g., schist, gneiss, etc.)
3.
Common textures
Foliations — planar fabric formed by layers or coplanar minerals
a. slaty cleavage - dull luster
b. phyllitic cleavage - shiny, pearly luster, but fine grained)
c. schistosity - shiny luster with visible micaceous minerals
d. crenulated cleavage/schistocity - refolded/kinked cleavage or schistocity
e. gneissic layering or banding – segregated into light granular & dark micaceous
bands
f. shear foliation - specific to fault/shear zones
g. mylonitic –fine grained rock with strongly foliated matrix minerals that wrap around
large porphyroblasts
Lineations — linear fabric formed by mineral alignment and intersections of planar features
a. mineral lineation
b. cleavage-bedding intersection
c. fold axis - bedding-axial plane intersection
Other rock textures
a. granular (or granofels)- medium to coarse granular texture
b. hornfelsic – fine-grained, granoblastic texture
c. porphyroblastic - large, well-shaped metamorphic minerals in finer matrix
d. poikiloblastic – large metamorphic mineral rich in inclusions of matrix material
2
e.
augen texture – “eye”-shaped alkali feldspar in gneissic rock